1. Field of the Invention
The present invention relates to a pulse wave data analyzing method, a system, and a program product for extracting information corresponding to electrocardiogram RR intervals out of pulse wave data concerning a living body.
2. Description of the Related Art
There is widely used, as an arrhythmia diagnostic method, a method of measuring an electrocardiogram of a subject, and using RR intervals in the electrocardiogram. The RR interval is a peak-to-peak interval of two consecutive R waves, which are the most frequently observed waves among P waves, Q waves, R waves, S waves, and T waves in one cycle of heartbeat in an electrocardiogram, as shown in FIG. 20. Expressing the RR intervals in a time-series manner enables to find out a heartbeat fluctuation. The heartbeat fluctuation is widely and clinically used as an index for assessing a biological control function of an autonomic nervous system involved in an organic activity of a living body.
The electrocardiogram measurement is generally performed, using a holter monitor equipped with electrodes for detecting cardiac activity potentials of a subject, or a like device. In use of the holter monitor, data measurement is conducted under a condition that after five or so electrodes are attached to the chest of the subject, and a data receiver is mounted on the waist of the subject in a medical institute, the subject performs a daily activity for one day or so. After the measurement is completed, the data stored in the receiver is outputted to a predetermined analyzer for an electrocardiographic waveform analysis to obtain the RR-intervals. The electrocardiogram measurement using the holter monitor requires the subject to perform a daily activity, with the electrodes being constantly attached to the body, which is stressful to the subject.
In view of the above, there is proposed a method of extracting information (hereinafter, called as “pulse wave RR intervals”) corresponding to the RR intervals, out of pulse wave data, without relying on an electrocardiogram. The term “pulse wave” in the specification and claims of the application means a change in volume of an arterial vessel resulting from blood inflow, which is observed as a waveform through a body surface, and is a vasomotor response. The pulse wave is associated with cardiac movements. Measuring peripheral vascular movements of a living body enables to indirectly obtain information substantially equivalent to RR intervals obtained based on an electrocardiogram.
Concerning the pulse wave RR interval detection, a first conventional art discloses a pulse wave RR interval measuring device for calculating pulse wave RR intervals by detecting a pulse wave of a subject along a time axis by a pulse wave sensor, and by analyzing pulse wave data obtained by a calculator to obtain peak values and peak points of time. A second conventional art discloses an arrhythmia detector for detecting an accurate pulse wave component by frequency-analyzing a pulse waveform detected by a pulse waveform detector and by removing a body motion component, and for detecting an arrhythmia, using the obtained frequency analysis result. Furthermore, a third conventional art discloses a pulse wave analyzing method of detecting pulse wave RR intervals by calculating a speed pulse waveform based on first derivation of a pulse waveform detected by a pulse wave sensor, and by detecting peaks of the speed pulse waveform.
The peaks of the pulse wave data, however, are not sharp peaks generally observed in R waves of an electrocardiogram. Also, the pulse wave data includes notches, reflected wave components, or the like, which correspond to sites of a pulse waveform where relatively small peak portions and bottom portions appear. The notches, the reflected wave components, or the like become noises in peak detection. Hereinafter, the notches, the reflected wave components, or the like are simply called as “notches” to simplify the description. It is not easy to automatically detect peaks corresponding to the R waves based on raw pulse wave data according to the conventional art. The first and the second conventional art do not particularly mention an approach of accurately extracting peaks out of pulse wave data. The pulse wave analyzing method disclosed in the third conventional art is capable of removing the notches to some extent. However, since the third conventional art adopts a method of removing the notches, using a threshold value obtained based on a mean value of peak-to-peak intervals, the method may misjudge an actual peak as a noise in the case where appearance of a peak is unpredictable as in the case of an arrhythmia.